The present invention relates generally to audio dynamics processors such as compressor expanders, limiters, noise reduction systems and more particularly concerns audio processors where the level or amplitude of an audio signal is dynamically altered in response to a generated voltage control signal. In particular the performance of downward expander type noise reduction systems designed for use with musical instruments will greatly benefit from this invention, however all dynamics processors may see improved performance by use of the present invention. All dynamics processors where a VCA (voltage controlled amplifier) or VCF (voltage controlled filter) is used to modify the amplitude or frequency of an audio signal in response to a DC control signal may see the above mentioned improved performance and transparency by use of this improved dynamics processing control system. As will be apparent to the skilled artisan the improvements of the invention can also be applied to all dynamics processors. One of the major benefits of the invention will be fully explained in conjunction with below threshold, downward expanders. The attack time of a dynamics processor can also see major benefit from various embodiments of the current invention. For example a compressor with fully program dependent attack and release time can be realized by use of the current invention. My current pending surround system patent application can also be further improved by use of the teachings contained herein. Surround systems, such as the one disclosed in my pending application, use numerous audio level detectors and generate control signals in response to a stereo input signal. By applying the continuously variable attack and release improvements disclosed in this application the subjective transparency of a surround system can be greatly improved. Below threshold, expander type noise gates have been commonly know and used for noise reduction in professional recording applications for many years. One of the most successful noise reduction systems for use with musical instruments, such as guitar, bass and keyboards, is a system commercially known as the “HUSH” noise reduction system. The HUSH noise reduction system uses a combination of low-level downward expansion and dynamically controlled low-pass filtering. The operation of the dynamically controlled low-pass filter portion of the HUSH is disclosed in my previous U.S. Pat. No. 4,696,044. When the HUSH system is used to provide noise reduction for instruments such as guitar, the downward expander provides the most important and most audible aspect of the performance of the system. Conversely, when using the HUSH noise reduction system with composite music, the dynamic filters provide the most critical aspect of the operation of the system. One of the most difficult applications for noise reduction is the removal, or suppression of the noise that is present in high gain guitar systems. My U.S. Pat. No. 4,881,047 discloses a noise reduction system specifically designed to suppress the gain noise of a high-gain distortion circuit. The system disclosed in the 047 patent will also greatly reduce the amount of audible hum present in a high-gain guitar distortion system by reducing the gain of the preamplifier distortion circuit. While the above disclosed systems and many of the previously available expander noise reduction systems have provided improvements in audio performance, they fall short of achieving optimal performance under all conditions. The prior art below threshold expander systems typically provide a preset or in some cases a user adjustable fixed slope release characteristic. In order to avoid audible distortion of the input signal or serious pumping side effects it is desirable to have a slow release time constant for the control signal. As is commonly know in the art, when using a very fast time constant for the control voltage, excessive ripple in the control signal will modulate the VCA of a dynamics processor thereby causing audible distortion, pumping or breathing. If the input signal contains low frequency components a fast time constant can cause modulation of each cycle of the audio signal, thereby causing undesirable and in some case very audible distortion. While slowing the release time of an expander will improve the above mentioned side effects a slow release time will also allow the noise floor to momentarily become audible when the input signal stops suddenly. This causes another objectionable side effect in the expander performance. Making the release time dynamically variable as described in my U.S. Pat. No. 4,881,047 can offer improvements in expander performance. While this is an improvement over typical below threshold expanders further improvements can be made by generating a more accurate, responsive and or continuously variable control signal. As described in the 047 patent, there is desire to have a control signal that will have a slow release time when the instrument signal has a long sustained decay and also provide a very fast release time when the musician is playing staccato, or abrupt notes. While the teachings of the 047 patent are an improvement over the prior art, further improvements in performance can be made by clamping the control signal so that the release time begins at a predefined voltage level, typically at a point equal to the expander threshold. Audio expanders typically use some form of level detection that converts the input audio signal to a DC control signal. The generated control signal typically has a predefined release time constant characteristic. When the input level drops below a user adjustable threshold point downward expansion will begin. The amount of expansion will increase as the input signal continues to drop further below the threshold point. In the prior art systems, the detection circuit will charge a timing capacitor well in excess of the predefined threshold point. The result is that when the input signal stops abruptly the prior art expander does not provide any reduction of the input signal until the timing capacitor voltage drops below the preset threshold point. This results in a “dead zone” where the control signal is decreasing but has no affect on the operation of the downward expander. While the teachings of the 047 patent show one way to improve this problem this design also suffers as a result of having a dead zone in the release response.
It is, therefore, a major object of the present invention to provide an improved audio dynamics processing control signal, for use in below threshold applications, that will eliminate this above described “dead zone” and allow the control signal to be immediately responsive to the input signal when it drops below the threshold point. Most of the prior art systems have a predefined or user adjustable release response. In the 047 patent an improved method of changing the release response time by switching to a fast time constant when the input signal drops rapidly is disclosed. While the teachings of the 047 patent are an improvement over the prior art, further improvements are possible by making the release time continuously variable and totally responsive to the envelope of the input signal. It is, therefore, a further objective of the present invention to offer a continuously variable release response that has a very slow release for long sustained signals and continuously varies the release slope to track the envelope of the input signal for more rapidly decaying signals. It is a further object of the present invention to provide a very fast release time when the input signal decays quickly or stops abruptly. Yet another objective of the present invention is to provide an improved audio dynamics processing control signal where both the attack and release time period is totally responsive to the short term envelope of the input signal and yet eliminates the audible side effects of the prior art systems.